The short answer would be no.  They're really decorative items, and in my view shouldn't be considered space heaters.  The basic reason for this is that all the hot air is going straight up.  So the convection flow is against you.  You're getting some radiant heat, but it's spreading out in all directions.  A typical Home Depot style fire pit is something like 30k - 50k BTU, which is going to heat up the front of you within something like a 5-6' radius circle around the fire pit.  Not great considering you'll be paying around $2.50 an hour if you're on propane.  (8-10 hours per tank, around $20.00 - $25.00 to fill up that bottle with current prices).  The dollar equation gets a bit better if you're on natural gas, which you'll want to be for most firepits with larger burners.  They can run 200,000 BTU plus on a large or custom fire pit.  And when you go up to that kind of burner, you'd like to think you get significantly more heated area, but alas.  I'd really like to do some testing on this, but my experience tells me that even a 200,000 BTU burner won't radiate heat more than about 6', giving a roughly 12' circle of radiant heat.  One therm of natural gas is up to about $1.80 nationwide, and a therm contains roughly 100,000 BTU.  So our 200k btu fire pit would cost us $3.60 per hour to run. 

The alternative?  Particularly for covered areas, you'll want a radiant electric or natural gas heater.  For comparison sake, let's look at a natural gas radiant patio heater.  Sunpak makes a good one, that uses 34,000 BTU per hour, and heats a 12x12 area.  It's much more efficient because the radiant heat is being directed, rather than just going out in all directions.  So now we're talking about a cost of roughly $.60 per hour to heat the same area.  You could also go electric, a typical 6,000 watt heater will heat something like 11 x 11, and cost would be in the $.75 per hour range.  Electric heaters also have a significantly lower installation cost.  You don't have to run gas, but you will have to run 240V power for most of them.  This makes a more economical option for larger areas, if you have a 10 x 20 covered patio for instance, two of those electric heaters will cover the entire area effectively. 

So why buy a firepit then?  Because sitting around an electric radiant heater might feel nice, but you get none of the atmosphere of the flame.  You want that gathering point, the mood of sitting around a fire.  So by all means buy one, they're really a lovely addition to the backyard.  Just be reasonable about the heating performance they can offer, and the cost-effectiveness they provide in those terms.  Of course we want to stay cozy, but we've got to keep that pocketbook in mind as well.